Method and apparatus for bonding optical disc substrates together, and method for supplying liquid material

ABSTRACT

The object of the present invention is to significantly inhibit the formation of voids between substrates bonded together when a liquid adhesive is supplied onto an optical disc substrate or when the optical disc substrate is bonded to another optical disc substrate. Accordingly, the present invention discloses a method for bonding two optical disc substrates together which comprises the steps of joining the optical disc substrates together with an adhesive and curing the adhesive, wherein the adhesive is supplied onto the optical disc substrate by an electric field formed between the adhesive-supplying nozzle, for supplying the adhesive onto the optical disc substrate, and the optical disc substrate, and the two optical disc substrates are then joined together and subjected to spun by a spinning process.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is a divisional application of U.S.non-provisional patent application Ser. No. 09/778,232, entitled “Methodand Apparatus for Bonding Optical Disc Substrates Together, and MethodFor Supplying Liquid Material,” filed on Feb. 6, 2001, herebyincorporated by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a method and an apparatus forforming a single optical disc substrate by bonding optical discsubstrates together and a method for supplying a liquid material.

[0004] 2. Description of the Related Art

[0005] When optical disc substrates are bonded together by an opticaldisc bonding apparatus using a liquid adhesive, it is important toensure that no void exists in the adhesive layer after the bonding ofthe substrates. Therefore, various proposals have heretofore beenconsidered, but all proposals have been unsatisfactory in that voidshaving a diameter of about 0.1 mm or larger, minute voids having adiameter of about 0.05 to 0.1 mm, or a mixture of these voids are formedbetween the optical disc substrates.

[0006] As a method for significantly minimizing such a problem, theapplicant of the present invention has applied the following invention,described in Japanese Patent Application No. Hei 10-257530. Theinvention will be described with reference to FIG. 8. Of two opticaldisc substrates A and B, a ring-shaped adhesive liquid film Ta is formedon the upper bonding surface of the lower optical disc substrate A. Onthe bonding surface of the upper optical disc substrate B, a pluralityof dot-shaped adhesive liquid films Th are formed in a circular shapehaving a diameter slightly larger than that of the ring-shaped adhesiveliquid film Ta. Thereafter, the two optical disc substrates A and B arebrought close together with the bonding surfaces thereof facing eachother, and the two optical disc substrates A and B are joined togetherbringing the ring-shaped adhesive liquid film Ta into contact with thedot-shaped adhesive liquid films Th. Then, the two optical discsubstrates A and B are spin-processed to spread out the adhesive liquidfilm Ta and the adhesive liquid films Th. The excess adhesive is spunoff from the substrates, and an adhesive layer having a uniform filmthickness is formed between the optical disc substrates A and B.

[0007] In this method, by properly bringing the ends of the dot-shapedadhesive liquid films Th, which are formed in a circular shape on theupper optical disc substrate B, into contact with the rim of thering-shaped adhesive liquid film Ta formed on the lower optical discsubstrate A, the development of particularly minute voids at the momentwhen these liquid films make contact with each other can be prevented.Further, since air between the liquid films is expelled when the contactportions between the liquid films expand over the entire liquid films,the occurrence of voids is lower at these points.

[0008] However, since it is still extremely difficult, even by thismethod, to make the contact area sufficiently small at the moment whenthe adhesive liquid film Ta and the adhesive liquid films Th contacteach other, the development of minute voids cannot be totally prevented.Further, voids may develop when the adhesive liquid film Ta or theadhesive liquid films Th make contact with the opposing optical discsubstrate B or A.

[0009] Further, since voids may also be formed when the adhesive liquidfilm Ta or the adhesive liquid films Tb are formed by supplying a liquidadhesive from an adhesive-supplying nozzle (not shown) onto the loweroptical disc substrate A or onto the upper optical disc substrate B, theformation of voids during this process also needs to be prevented.

[0010] The present invention has been invented in view of the abovecircumstances. The object of the present invention is to provide amethod and an apparatus for bonding optical disc substrates together,which rarely or never generate voids between the optical disc substrateswhen a liquid adhesive is supplied onto the optical disc substrate orthe like, or when the two optical disc substrates or the like suppliedwith an adhesive are bonded together.

SUMMARY OF THE INVENTION

[0011] The present invention relates to a method for bonding two opticaldisc substrates together which comprises the steps of joining theoptical disc substrates together with an adhesive and curing theadhesive, in which the adhesive is supplied onto the optical discsubstrate by an electric field formed between an adhesive-supplyingnozzle, for supplying the adhesive onto the optical disc substrate, andthe optical disc substrate, and the two optical disc substrates are thenjoined together and spun by a spinning process.

[0012] When the adhesive is supplied onto one of the two optical discsubstrates in the shape of a ring, the other optical disc substrate maynot have the adhesive applied thereon at all, may have the adhesive filmformed over almost the entire surface, or may have the adhesive formedinto dots at a relatively small spacing in a circular shape.

[0013] When the adhesive is supplied as dots at a relatively smallspacing in a circular shape on one of the two optical disc substrates,the other optical disc substrate may not have the adhesive appliedthereon at all or may have the adhesive film formed over almost theentire surface.

[0014] The present invention also relates to an apparatus for bondingoptical disc substrates together by joining the two optical discsubstrates together with an adhesive and curing the adhesive, whichcomprises an adhesive-supplying nozzle for supplying the adhesive ontothe optical disc substrate, an electrode means placed in contact with orin the vicinity of the surface of the optical disc substrate which isopposite the surface which faces the adhesive-supplying nozzle, and anelectric power supply for generating an electric field between theelectrode means and the adhesive-supplying nozzle.

[0015] The adhesive-supplying nozzle comprises a single nozzle or twonozzles separated from each other by almost 180 degrees, is placed overthe optical disc substrate nearly perpendicular thereto with its tip(s)pointing downward, and forms a ring-shaped adhesive liquid film on theoptical disc substrate which spins relative to the nozzle(s).

[0016] The adhesive-supplying nozzle may comprise, for example, aplurality of nozzles placed at an approximately uniform spacing in acircular shape, may be placed under the optical disc substrate nearlyperpendicular thereto with their tips pointing upward, and may supplydot-shaped adhesive liquid films onto the underside of the optical discsubstrate.

[0017] In this case, the electric field generated by the electric powersupply may be an alternating-current or a direct-current electric field.However, the alternating-current electric field is preferable.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]FIG. 1A is a front view of optical disc substrate whichillustrates an embodiment of the method for bonding optical discsubstrates of the present invention.

[0019]FIG. 1B is a front view of optical disc substrate whichillustrates an embodiment of the method for bonding optical discsubstrates of the present invention.

[0020]FIG. 1C is a front view of optical disc substrate whichillustrates an embodiment of the method for bonding optical discsubstrates of the present invention.

[0021]FIG. 1D is a front view of optical disc substrate whichillustrates an embodiment of the method for bonding optical discsubstrates of the present invention.

[0022]FIG. 2 is an upper perspective view which illustrates anembodiment of the method and the apparatus for bonding optical discsubstrates together according to the present invention.

[0023]FIG. 3 is a cross-sectional view which illustrates an embodimentof the method and the apparatus for bonding optical disc substratestogether according to the present invention.

[0024]FIG. 4 is a cross-sectional view which illustrates an embodimentof the method and the apparatus for bonding optical disc substratestogether according to the present invention.

[0025]FIG. 5A is a front view and a side view of optical disc substrateswhich stepwise illustrates an embodiment for bonding optical discsubstrates together according to the present invention.

[0026]FIG. 5B is a front view and a side view of optical disc substrateswhich stepwise illustrates an embodiment for bonding optical discsubstrates together according to the present invention.

[0027]FIG. 5C is a front view and a side view of optical disc substrateswhich stepwise illustrates an embodiment for bonding optical discsubstrates together according to the present invention.

[0028]FIG. 5D is a front view and a side view of optical disc substrateswhich stepwise illustrates an embodiment for bonding optical discsubstrates together according to the present invention.

[0029]FIG. 6 is a front view and a side view of optical disc substrateswhich illustrate an embodiment for bonding optical disc substratestogether according to the present invention.

[0030]FIG. 7 is a circuit diagram which illustrates an embodiment forbonding of optical disc substrates together according to the presentinvention.

[0031]FIG. 8 is a front view and a sectional view of optical discsubstrates which illustrates a conventional method for bonding opticaldisc substrates together.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0032] A description of the principle of the present invention willfirst be given. The present invention is based on a finding that when anelectric field, especially an alternating-current electric field, isgenerated between an adhesive-supplying nozzle and an optical discsubstrate at the time of supplying a liquid adhesive from theadhesive-supplying nozzle onto the optical disc substrate, a contactarea of the liquid film of the adhesive supplied onto the optical discsubstrate which makes contact with the other optical disc substratedirectly or when the adhesive liquid film is formed thereon for thefirst time, can be smaller, and the less likely it is that voids areformed in compliance with the contact area is smaller.

[0033] It is believed that because the electric field becomes quitelarge immediately before the liquid film of the adhesive first makescontact, an end of the liquid film of the adhesive is tapered off,thereby making the contact area smaller.

[0034] Generally known digital versatile discs (DVD) are classifiedinto: a single-sided single-layer optical disc in which only one of theoptical disc substrates to be bonded together has a recording layercomprising pits and a reflective layer; a double-sided single-layeroptical disc in which both of the optical disc substrates to be bondedtogether have a recording layer; a single-sided double-layer opticaldisc in which the reflective layer of one of the optical disc substratesto be bonded together is a translucent film; a combination of thesingle-sided single-layer optical disc and the single-sided double-layeroptical disc described above; and a double-sided double-layer opticaldisc comprising two single-sided double-layer optical discs bondedtogether. The present invention can be applied to the production of anyof these various types of DVD's.

[0035] Before a description of the embodiments of the present inventionis given to, the shapes and combinations of liquid adhesives on anoptical disc substrate from which the desired effects can be expectedwhen the present invention is applied will be described by taking anoptical disc substrate having a hole in the center as an example. InFIG. 1A, on the optical disc substrate denoted by the mark A, anadhesive liquid film Ta is formed in the shape of a ring around acentral hole H. In FIG. 1B, on the optical disc substrate A, dot-shapedadhesive liquid films Th are formed into dots at a small spacing in acircular shape having a central hole H in the center. In FIG. 1C, on theoptical disc substrate A, a flat adhesive liquid film Tc is formed overalmost the entire surface of the substrate except for a predeterminedcircular area having a central hole H in the center. Further, in FIG.1D, the optical disc substrate A has no adhesive liquid film formedthereon.

[0036] Next, combinations of the substrates A denoted in FIGS. 1A to 1D,with which the effect of the present invention can be obtained, will bedescribed. As for the substrate A denoted in FIG. 1A, a combination witha substrate similar to the substrate A denoted in FIG. 1A or with any ofthe substrates A denoted in FIGS. 1B to 1D can obtain the desired effectof the present invention by employing the voltage-applying methodaccording to the present invention. As for the substrate A denoted inFIG. 1B, similarly, a combination with a substrate similar to thesubstrate A denoted in FIG. 1B or with any of the substrates A denotedin FIGS. 1A to 1D can obtain the desired effect of the present inventionby employing the voltage-applying method according to the presentinvention. On the other hand, a combination of the substrate A denotedin FIG. 1C and the substrate A denoted in FIG. 1D and a combination ofthe substrate A denoted in FIG. 1C and a substrate similar to thesubstrate A denoted in FIG. 1C cannot produce the desired effect of thepresent invention even by employing the voltage-applying methodaccording to the present invention.

[0037] A description of the first embodiment of the present inventionwill be given with reference to the drawings.

[0038]FIG. 2 is a diagram for illustrating an embodiment in which aring-shaped adhesive liquid film Ta as denoted in FIG. 1A is formed. Anadhesive-supplying nozzle 1 for supplying a liquid adhesive to theoptical disc substrate A is a small-diameter pipe made of an ordinarymetallic material, by which a general liquid-supplying operation isperformed. The adhesive-supplying nozzle 1 is connected to one of theterminals of an alternating-current power supply 2 as well as to theground potential, and an electrode means 3 which serves as an electrodeon a support stage is connected, via a switch 4, to the other terminalof the alternating-current power supply 2. Therefore, when the switch 4is ON and a sinusoidal voltage from the alternating-current power supply2 is applied between the adhesive-supplying nozzle 1 and the electrodemeans 3, an alternating-current electric field is generatedtherebetween. By continuously supplying a predetermined amount of aliquid adhesive from the adhesive-supplying nozzle 1 onto the bondingsurface of the disc substrate A while the support stage is rotatedalmost 360° at a fixed speed while the alternating-current electricfield is generated, a ring-shaped adhesive liquid film Ta as denoted inFIG. 1A is formed. In this case, end of the liquid film of the adhesiveis tapered off due to the effect of the alternating-current electricfield and the contact area becomes smaller, whereby voids are lesslikely to be formed between the optical disc substrate A and the liquidfilm Ta.

[0039] The amount of the sinusoidal voltage applied cannot be generallydetermined because it depends on the rotation speed of the optical discsubstrate A or the adhesive-supplying nozzle 1, the ejection speed ofthe liquid adhesive, and properties of the liquid adhesive such asresistivity and viscosity. In the embodiment, the sinusoidal voltagehaving a peak value of about 1 kV and a frequency of 500 Hz was used. Inthis case, the amount of the sinusoidal voltage applied is preferably assmall as possible in order to minimize the risk of occurrence ofelectrical discharge but large enough to achieve the desired object. Ithas been found that by setting the frequency of the applied sinusoidalvoltage equal to 50 Hz and above when the ring-shaped adhesive liquidfilm Ta as denoted in FIG. 1A is formed, the amount of the appliedsinusoidal voltage can be decreased while the desired object to preventor reduce the formation of voids is achieved. Thus, the frequency of thesinusoidal voltage applied is preferably equal to 50 Hz and above.

[0040] Next, an embodiment, in which dot-shaped adhesive liquid films Thas denoted in FIG. 1B are formed, will be described with reference toFIG. 3. A base portion 5, which is made of a metallic material or asynthetic resin material whose surface is covered with an electricalinsulating coating, has a ring-shaped peripheral wall portion 6 providedalong the outermost periphery of one surface of the base portion for thepurpose of storing an excess of the adhesive supplied from theadhesive-supplying nozzle 1. The adhesive-supplying nozzle 1 comprises aring-shaped common nozzle portion 1 a formed around the center of thebase portion 5 and nozzle portions 1 b formed at an approximatelyuniform spacing in the circumferential direction of the common nozzleportion 1 a, and is made of a metal such as stainless steel. The numbersof the nozzle portions 1 b are the same as the numbers of dots in thedot-shaped adhesive liquid films Th as denoted in FIG. 1B for example.Further, the base portion 5 has a liquid-supplying channel 7 forsupplying a liquid adhesive to the adhesive-supplying nozzle 1. At thecenter of the base portion 5, a support 9 is fixed which supports acenter pin 8 which is inserted into the central hole H of the opticaldisc substrate A for positioning the optical disc substrate A.

[0041] A support means 10 supports the optical disc substrate in orderto move the optical disc substrate not only vertically but alsohorizontally as required, and comprises a ring-shaped plate ordisc-shaped electrode portion 11 and a positioning means 12 which holdsthe center pin 8 on the main surface thereof. This support means 10 isconnected to a driving mechanism for moving the optical disc substratevertically or in other directions. Further, although not shown, asuction path and the like, for selectively holding the optical discsubstrate by suction, is formed on the underside of the electrodeportion 11. The electrode portion 11 is connected, via the switch 4, toone of the terminals of the alternating-current power supply 2, and theadhesive-supplying nozzle 1 is connected not only to the groundpotential but also to the other terminal of the alternating-currentpower supply 2. Further, the center pin 8 and the support 9 are notnecessarily required, and it is also by all means acceptable to detectvia a sensor (not shown) whether the support means 10 is movedhorizontally to a predetermined position over the adhesive-supplyingnozzle 1, in order to stop the horizontal movement and then move thesupport means 10 downward.

[0042] In the case of forming the liquid films Th, when the supportmeans 10 holds the optical disc substrate A by suction at the differentposition from the position shown in FIG. 3, the support means 10 movesupper area of the illustration shown in FIG. 3 and then starts to movedownward. By flipping the switch 40N during this process, the sinusoidalvoltage from the alternating-current power supply 2 is applied to thewhole adhesive-supplying nozzle 1 and the electrode portion 11 of thesupport means 10, and thereby an alternating-current electric field isgenerated between them. When the alternating-current electric field isgenerated, the optical disc substrate A is stopped about 0.4 to 2 mmabove the tips of the adhesive-supplying nozzle 1, and the liquidadhesive from the adhesive-supplying nozzle is applied to the undersideof the optical disc substrate A, as exemplified by the substrate denotedin FIG. 1B. Thereafter, the optical disc substrate A is brought upwardand removed to proceed to the next step. Incidentally, as a safetymeasure, an elastic material 9 a is fixed on the upper surface of thesupport 9 to prevent the optical disc substrate A from approaching thetips of the adhesive-supplying nozzle 1 and further extends to a pointabout 0.4 mm away from the tips. The upper surface of the elasticmaterial 9 a is about 0.4 mm above the tips of the adhesive-supplyingnozzle 1. In other words, the elastic material 9 a serves as a stopper.

[0043] When a liquid adhesive is applied from the adhesive-supplyingnozzle 1 to the underside of the optical disc substrate A when thealternating-current electric field is generated described above, theintensity of the alternating-current electric field becomes relativelyhigh because the spacing between the adhesive-supplying nozzle 1 and theelectrode portion 11 of the support means 10 becomes very small. As aresult, when viewed microscopically, immediately before application, theadhesive at the end of the adhesive-supplying nozzle 1 tapers offupward, and the contact area at the time of first making contact withthe optical disc substrate A is sufficiently small. This is the reasonwhy voids are rarely formed. As a result, an adhesive liquid film can beobtained having the pattern as denoted in FIG. 1B, in whichsubstantially no voids are formed even by the method and apparatuspresented in the present embodiment.

[0044] In the present embodiment as well, the amount of appliedsinusoidal voltage cannot be generally determined because the amount ofapplied sinusoidal voltage depends on the ejection speed of the liquidadhesive, properties of the liquid adhesive such as resistivity andviscosity, the capacitance between the electrodes, and the like.However, a sinusoidal voltage having a peak value of its about 400 V orhigher is required. In the present embodiment, a sinusoidal voltagehaving a peak value of about 900 V and a frequency of 4 kHz or higher,and in consideration of the audio-frequency range, a sinusoidal voltageof 20 kHz was used.

[0045] Next, the bonding of the optical disc substrate A to the opticaldisc substrate B will be described with reference to FIGS. 1, 3, and 4by considering the case where the optical disc substrate A is one havinga ring-shaped adhesive liquid film as denoted in FIG. 1A, and theoptical disc substrate B is one having no adhesive liquid film formedthereon as denoted in FIG. 1D.

[0046] The lower optical disc substrate A having the ring-shapedadhesive liquid film Ta as denoted in FIG. 1A is mounted on a support 15of a bonding apparatus. The support 15 has a center axis 15A protrudingat the center. The side wall of the center axis 15A is divided into aplurality of sections, through which chuck pawls, which will bedescribed later, can pass. Further, the support 15 has a ring-shapedelectrode portion 16 to which one of the terminals of analternating-current power supply 18 is connected via a power supply wire17. Meanwhile, the upper optical disc substrate B having no adhesiveliquid film formed thereon is supported by a support means 19. Thesupport means 19 is a disc-shaped member made of a conductive materialsuch as stainless steel, which serves as one of the electrodes, and hasa general suction mechanism (not shown) by which the support means 19holds the upper surface of the upper optical disc substrate B bysuction. Further, the support means 19 is connected to a transfer arm(not shown) that is horizontally spinnable at a certain angle and iselectrically grounded through the transfer arm.

[0047] At the center of the support means 19, a chuck means 20 having anaxis whose center is aligned with the center of the center axis 15A ofthe support 15 is fixed. The chuck means 20 has three chuck pawls 20Awhich can change the diameter of the chuck means 20 by an externalsignal. The chuck pawls 20A expand the diameter in the central holes ofthe optical disc substrates A and B in order to hold the inner walls ofthe optical disc substrates A and B when the optical disc substrates Aand B are transferred to another position while maintaining thesubstrate B on the substrate A.

[0048] The operation of this mechanism will be described with referenceto FIGS. 5 and 6 in addition to the above FIGS. 1 to 4. The loweroptical disc substrate A is mounted on the support 15, and in thisstate, the continuous ring-shaped liquid film Ta is formed on the uppersurface of the optical disc substrate A as described in the aboveembodiment. Then, when the optical disc substrates A and B are oppositeeach other, a sinusoidal voltage is applied between the electrodeportion 16 of the support 15 and the support means 19 by thealternating-current power supply 18 to generate an alternating-currentelectric field between the optical disc substrates A and B. Next, thesupport 15 is elevated by moving the lifting shaft 21 fixed on theunderside of the support 15 upward by means of a driving mechanism (notshown) such as a cylinder device, whereby the spacing between theoptical disc substrates A and B becomes smaller as denoted in FIG. 5Aand the top of the liquid film Ta eventually makes contact with theunderside of the optical disc substrate B as denoted in FIG. 5B. Duringthis approaching process of the liquid film Ta, since the intensity ofthe electric field between the optical disc substrates A and B becomesstronger as the spacing between them becomes smaller, the top of theliquid film Ta tapers off upward by suction force caused by the electricfield, and the tapered top of the liquid film Ta makes contact with theunderside of the optical disc substrate B first. Therefore, the area ofthe top of the liquid film Ta at the moment when the liquid film Tamakes contact with the optical disc substrate B becomes significantlysmaller than that of the prior art. The initial state of the contact ismagnified and shown in FIG. 6. The liquid film Ta which makes contactwith the optical disc substrate B spreads not only in thecircumferential direction but also in the radial direction between theoptical disc substrates A and B to form a circle as denoted in FIGS. 5Cand 5D.

[0049] Next, the chuck means 20 is operated by the external signal, andthe chuck pawls 20A move to expand the diameter of the chuck means 20 inthe central holes of the optical disc substrates A and B so as to holdthe inner walls of the optical disc substrates A and B. With this statemaintained, the support 15 is brought down by moving the lifting shaft21 downward, and the optical disc substrates A and B are held by thechuck means 20 and supported by the support means 19. In reality, theadhesive liquid film between the optical disc substrates A and B in thisstate spreads out much more widely than shown in FIG. 5, and minutevoids or larger voids are not observed when the liquid film is observedthrough the optical disc substrates A and B. Thereafter, the supportmeans 19 is spun by a spinning means (not shown) and transfers theoptical disc substrates A and B to a spinner (not shown).

[0050] That is, in the present embodiment, since an electric field isgenerated when the liquid adhesive is supplied onto the optical discsubstrate A or when the optical disc substrates A and B are bondedtogether, the adhesive can make contact with the optical disc substrateB in a very preferable condition, and the formation of voids between theoptical disc substrates A and B bonded together can be significantlyinhibited.

[0051] When the optical disc substrates A and B are bonded together, itis difficult to make them sufficiently parallel to each other across thewhole surface, and it is also extremely difficult to make the thicknessof the adhesive liquid film Ta uniform. Therefore, when the adhesiveliquid film Ta is observed microscopically at the time of contactingwith the optical disc substrate B, the adhesive condition of theadhesive liquid film Ta is not actually uniform. Therefore, when adirect-current voltage is applied, the first portion of the liquid filmwhich makes contact is wetted due to the effect of applying the voltagebut the second and later portions of the liquid film which make contactare not wetted as well as the first portion. This is because thepositive and negative electric charges induced between the optical discsubstrates A and B through the resistance R (to be described later) ofthe adhesive start to neutralize from the moment when the first portionof the liquid film makes contact, whereby the voltage between theoptical disc substrates A and B decreases and the effect of applying thevoltage is therefore weakened to some extent. Thus, the sinusoidalvoltage was applied in the present embodiment.

[0052] When the sinusoidal voltage is applied, the electrode portion 16of the support 15, the reflective film (not shown) of the optical discsubstrate A and the insulating material of the optical disc substrate Aform a first capacitance and exhibit an impedance Z1 as shown in FIG. 7.The gap between the reflective film of the optical disc substrate A andthe reflective film (not shown) of the optical disc substrate B forms asecond capacitance and exhibits an impedance Z2 as shown in FIG. 7.Further, the reflective film of the optical disc substrate B, thesupport means 19 and the insulating material of the optical discsubstrate B interposed between them form a third capacitance and exhibitan impedance Z3 as shown in FIG. 7. Furthermore, in FIG. 7, the gapbetween the reflective film of the optical disc substrate A and thereflective film of the optical disc substrate B is denoted by a switch Sand the resistance of the adhesive is denoted by R; these are connectedin parallel to the impedance Z2.

[0053] Since all of the impedances Z1 to Z3 tend to become small inresponse to an increase in the frequency f of a voltage to be applied(for example, Z1=½πfC3 and Z3=½πfC1, on the proviso that Z1 and Z3 areabsolute values), the impedances Z1 to Z3 can be made small by applyinga sinusoidal voltage having an appropriate frequency between the opticaldisc substrates A and B. Therefore, when the frequency f is determinedsuch that the values of the impedances Z1 to Z3 are to be equal to orless than the resistance R of the adhesive, the voltage V2 between thereflective film of the optical disc substrate A and the reflective filmof the optical disc substrate B is hardly affected by the resistance R.

[0054] That is, when the sinusoidal voltage is applied between theoptical disc substrates A and B, by setting the frequency f of thesinusoidal voltage properly, the voltage V2 between the reflective filmof the optical disc substrate A and the reflective film of the opticaldisc substrate B hardly decreases even when the adhesive liquid film Tamakes contact with the optical disc substrate B. This indicates that theeffect of applying the voltage is still maintained even when a pluralityof different portions of the adhesive liquid film Ta make contact atdifferent times.

[0055] In this case, when conditions such as the thicknesses of theoptical disc substrates A and B, the dielectric constants and theresistivity of the adhesive are taken into consideration, the effect ofapplying the voltage is large when the frequency f of the sinusoidalvoltage to be applied is 50 Hz or higher.

[0056] According to the present embodiment, since the top of the liquidfilm Ta is tapered by the alternating-current electric field and theadhesive makes contact with the optical disc substrate B at the top, theoccurrence of minute voids which are likely to be formed at the time ofbonding is sufficiently inhibited. Further, since the liquid film Taquickly spreads out not only in the circumferential direction but alsoin the radial direction between the optical disc substrates A and Bwhose surfaces are charged with positive and negative electric charges,no air is trapped therebetween in this process. Therefore, voids whichare larger in diameter than the minute voids are also not formed in theadhesive layer which has been spread out thinly and uniformly betweenthe optical disc substrates A and B by the spinning process. Further,when the voltage to be applied is a sinusoidal voltage, voltage must beapplied based on the mean value of the sinusoidal voltage since theabsolute mean value affects the effect the voltage application. Further,the waveform of the sinusoidal voltage is not limited to a sinusoidalwave and can be a positive and negative alternating waveform such as arectangular wave, a trigonal wave or a sinusoidal wave having periodswhen the voltage is not applied.

[0057] Further, when the substrates denoted in FIG. 1A are bondedtogether, when the substrate denoted in FIG. 1A, 1C or 1D are bondedtogether, or even when the substrate denoted in FIG. 1B and thesubstrate denoted in FIG. 1A, 1C or 1D are bonded together, the presentinvention can be applied to these cases exactly in the same manner as inthe present embodiment, and the same effect can be obtained. Thus,descriptions therefor will be omitted.

[0058] Further, the present invention can be applied not only to thecase where optical disc substrates are flat but also to the case whereoptical disc substrates having a curved surface, such as lenses, arebonded together, and the same effect can be obtained. Further, althoughthe case where a single adhesive-supplying nozzle is used has beendescribed in the embodiment shown in FIG. 2, two adhesive-supplyingnozzles can be used which are separated from each other by 180 degreesand supply adhesives on the optical disc substrate simultaneously whilethe optical disc substrate is rotated about a half turn. Further,instead of the optical disc substrate, the adhesive-supplying nozzle(s)can be rotated at a fixed speed. Still further, there is also a casewhere the same effect can be obtained by applying a direct-currentvoltage. Incidentally, the switch 4 is shown in FIGS. 2 and 3 forillustrating the ON and OFF states of the sinusoidal voltage and issubstituted with the switching element of the primary circuit of thealternating-current power supply in an actual apparatus.

[0059] Although the supplying of the adhesive in the process of bondingoptical disc substrates together has been described in the aboveembodiment, similarly, it is common practice in the production ofcompact disc (CD) that a liquid material is supplied onto a disc in theshape of a ring, and the disc is then spun at a high speed to form aprotective film or recording film having less bubbles. It has beenconfirmed that exactly the same effect as obtained in the case of theabove adhesive can be obtained when the present invention is applied tothe production of compact disc. That is, just as is shown in FIG. 2,when a liquid material capable of forming a ring-shaped protective orrecording film is supplied onto a CD substrate, a voltage is appliedbetween a supply nozzle for supplying the liquid material and the CDsubstrate, whereby the wettability between the liquid material and theCD substrate improves, resulting in that voids which are likely to beformed between the liquid material and the CD substrate can be almosteliminated. This can also be applied to the case where a liquid materialfor forming a protective film or a resist film is supplied as dotsaround the center of the surface of a polygonal or circular glass plate,a semiconductor wafer, or a plate-shaped object as a lens has a curvedsurface.

[0060] Further, even when the liquid material supplied, as describedabove, is not sandwiched between the substrates but is spread out in asubstantially uniform thickness by using a spin-coating device such as ageneral spin coater, a coating film of high quality having few voids canbe obtained by applying the present invention. This case will bedescribed in more detail. When a liquid material is supplied from thesupply nozzle by a voltage applied between the nozzle and the CDsubstrate as described above and an excess of the liquid material isthen spun off from the substrate by high-speed spinning to form acoating film having a desired thickness at the position of coating thefilm or another position, a support stage (not shown), spinning at ahigh speed with the CD substrate thereon, is provide acting as a lowerelectrode, an upper electrode plate is placed above the CD substrate,and the above-described sinusoidal voltage is applied to the electrodes,whereby the wettability to the liquid material of the CD substrateimproves, resulting in a great reduction in the number of voids whichare likely to be generated between the liquid material and the CDsubstrate. Note that the voltage can be applied in the same manner as inthe above embodiment.

What is claimed is:
 1. An apparatus for bonding two optical discsubstrates together by joining the optical disc substrates together withan adhesive and by curing the adhesive, which comprises anadhesive-supplying nozzle for supplying the adhesive onto at least oneof the optical disc substrates, an electrode means placed in contactwith or in the vicinity of the surface of the optical disc substratewhich is opposite to the surface which faces the adhesive-supplyingnozzle, and an electric power supply for generating an electric fieldbetween the electrode means and the adhesive-supplying nozzle.
 2. Theapparatus according to claim 1, wherein the adhesive-supplying nozzlecomprises a single nozzle or two nozzles placed separated from eachother by almost 180 degrees away from each other, is placed over theoptical disc substrate nearly perpendicular thereto with its tip(s)pointing downward, and forms a ring-shaped adhesive liquid film on theoptical disc substrate which spins relative to the nozzle(s).
 3. Theapparatus according to claim 1, wherein the adhesive-supplying nozzlecomprises a plurality of nozzles placed at an approximately uniformspacing in a circular shape, is placed under the optical disc substratenearly perpendicular thereto with their tips pointing upward, andsupplies dot-shaped adhesive liquid films onto the underside of theoptical disc substrate.
 4. The apparatus according to claim 1, whereinthe electric power supply generates an alternating-current electricfield.
 5. The apparatus according to claim 2, wherein the electric powersupply generates an alternating-current electric field.
 6. The apparatusaccording to claim 3, wherein the electric power supply generates analternating-current electric field.
 7. The apparatus according to claim1, wherein the electric power supply generates a direct-current electricfield.
 8. The apparatus according to claim 2, wherein the electric powersupply generates a direct-current electric field.
 9. The apparatusaccording to claim 3, wherein the electric power supply generates adirect-current electric field.
 10. The apparatus according to claim 1,wherein the electric power supply generates the electric field betweenthe electrode means and the adhesive-supplying nozzle, so as to taper anend of a liquid film of the adhesive which is supplied by theadhesive-supplying nozzle toward the optical disc substrate in order toreduce an initial contact area between said end of the liquid film ofthe adhesive and the optical disc substrate for preventing generation ofvoids in said adhesive.
 11. An optical disc bonding apparatus forjoining two optical disc substrates together with an adhesive andforming an optical disc, the apparatus comprising: a device for formingan electric field between said two optical disc substrates, wherein anend of a liquid film of said adhesive which is supplied onto one of saidoptical substrates tapers, thereby making a contact area between saidend of said liquid film and the other of said optical disc substrateswhich is opposed to said end smaller by an effect of said electricfield, and generation of voids in said liquid film is prevented.
 12. Anoptical disc bonding apparatus for joining two optical disc substratestogether with an adhesive and forming an optical disc, the apparatuscomprising: a device for forming an electric field between said twooptical disc substrates, wherein an end of a liquid film of saidadhesive which is supplied onto one of said optical substrates tapers,thereby making a contact area between said end of said liquid film andother liquid film of said adhesive which is supplied onto the other ofsaid optical disc substrates and is opposed to said end smaller by aneffect of said electric field, and generation of voids in said liquidfilm is prevented.